This invention relates to an apparatus for extracting features of skin pattern images and an image processor for smoothing and enhancing features of the skin pattern images making use of the extracted features. More particularly, the invention relates to an apparatus for extracting such features as ridge line directions, ridge line pitches or their combination of skin pattern images for fingerprint verification, fingerprint classification or palm print verification, and an image processor for processing skin pattern images for enhancing ridge lines by eliminating noises of the images maling use of the extracted features.
As a prior art, there is a Japanese patent application entitled "An Apparatus for Striped Pattern Extraction" and laid open as a Provisional Publication No. 77138/76. In the prior art, which is hereafter called a first prior art, an apparatus is disclosed for eliminating noises from such striped images as fingerprints or tree rings and extracting both stripe patterns and directional patterns of the striped images.
FIG. 14 shows the principle of the apparatus of the first prior art. An image in a subregion 100 with its center at a small square 101 is represented by a set of pixel data. Each small square in the subregion 100 represents a pixel, and a numeral 1 or a blank in a small square means corresponding pixel value 1 (black) or 0 (white). Filter data sets 102, 103, 104, 105, and 106 for masking are prepared corresponding to no direction d0 and four directions d1, d2, d3, and d4, respectively. By calculating their sum of products with the pixel data of the image exampled in FIG. 14, values +5, 0, +11, +3 and -7 are obtained corresponding to each direction d0 to d4, respectively.
From these values, ridge direction at pixel 101 of the image is determined to the direction d2 which gives the maximum value +11 and stripe pattern value at the pixel 101 is determined to the maximum value +11.
Thus, stripe patterns and directional patterns are extracted as features of striped images in the first prior art.
Smoothing of extracted features is sometimes necessary for decreasing noise influence on the features. For instance, a smoothing method is disclosed in a Japanese patent application entitled "Method of Smoothing a Ridge Direction Pattern and an Equipment used therefor" and laid open as a Provisional Publication No. 18195/93. This smoothing method, hereafter called a second prior art, is based on a minimum energy principle.
From ridge direction data extracted with their reliability coefficients, a smoothed ridge direction pattern is determined in the prior art, wherein it is assumed that direction data extracted with higher reliability needs more energy to be smoothed into a direction pattern and that it needs more energy to smooth direction data into a direction pattern the more different from neighboring direction patterns. Therefore, a direction pattern which gives a minimum value of an evaluation function, linear summation of those energies, is determined as the smoothed direction pattern of a subregion.
However, pixel values of a striped image vary periodically in a direction perpendicular to the stripes. Therefore, when stripe phase of a striped image is nearly orthogonal to the stripe phase of the filter data for masking used in the first prior art, the sum of products results nearly 0 even when both directions coincide with each other. This means that the extracted direction patterns and, consequently, the extracted stripe patterns become unreliable since they are influenced by noise data, making precise ridge line pitch extracion difficult.
In the second prior art, smoothing of ridge direction patterns is performed based on extracted ridge directions. So, when extracted ridge direction data are not reliable, precise smoothing of ridge direction patterns and, consequently, precise smoothing of ridge line pitches can not be performed.